JSF Mongo nested documents set: insert causes error

Test Spring Roo application has two documents:

@RooJavaBean
@RooToString
@RooJpaActiveRecord
public class Amount {

private Long base;

@ManyToMany(cascade = CascadeType.ALL)
private Set<Cat> cats = new HashSet<Cat>();
}

and

@RooJavaBean
@RooToString
@RooJpaActiveRecord
public class Cat {

private String category;
}

Application also has a Roo-generated JSF (Primefaces) interface, and insert dialog looks as fallows:

SelectManyMenu catsCreateInput = (SelectManyMenu) application.createComponent(SelectManyMenu.COMPONENT_TYPE);
catsCreateInput.setId("catsCreateInput");
catsCreateInput.setConverter(new CatConverter());
catsCreateInput.setValueExpression("value", expressionFactory.createValueExpression(elContext, "#

{amountBean.selectedCats}

", List.class));
UISelectItems catsCreateInputItems = (UISelectItems) application.createComponent(UISelectItems.COMPONENT_TYPE);
catsCreateInputItems.setValueExpression("value", expressionFactory.createValueExpression(elContext, "#

{catBean.allCats}

", List.class));
catsCreateInput.setRequired(false);
catsCreateInputItems.setValueExpression("var", expressionFactory.createValueExpression(elContext, "cat", String.class));
catsCreateInputItems.setValueExpression("itemLabel", expressionFactory.createValueExpression(elContext, "#

{cat}", String.class));
catsCreateInputItems.setValueExpression("itemValue", expressionFactory.createValueExpression(elContext, "#{cat}

", Cat.class));
catsCreateInput.getChildren().add(catsCreateInputItems);
htmlPanelGrid.getChildren().add(catsCreateInput);

When I try to insert new Amount, I get:

Cats:: Validation Error: Value is not valid

I tested the same case using Roo Spring MVC web front-end, everything works. So, it's a problem of Roo JSF configuration. JSF front-end also doesn't work with PostgreSQL (the same data schema), but MVC works perfectly.